There exists a fundamental gap in our understanding of how intestinal microbiota impact the ability of their animal hosts to harvest energy-rich dietar fats. Moreover, the microbial and nutritional signals and responsive host transcriptional mechanisms that control host gene expression in the intestinal epithelium, remain unresolved. Our long-term goal is to understand the mechanisms underlying host-microbe interactions in the intestine and how those interactions impact human health and disease. The overall objectives of this project are to define the mechanisms by which members of the intestinal microbiota promote absorption of dietary fats, and how microbe-induced alterations in fat metabolism regulate gene expression programs in intestinal epithelial cells (IECs). Our preliminary studies in gnotobiotic zebrafish and mice reveal a novel role for microbiota in promoting absorption of long- and medium-chain fatty acids (FA) in intestinal epithelial cells, and implicate the FA-regulated transcription factor family Hepatic nuclear factor 4 (Hnf4) in mediating IEC transcriptional responses to microbiota. The proposed research will address the central hypothesis that soluble products from gut bacteria stimulate host absorption of dietary FA and reduce activity of Hnf4 transcription factors to reduce expression host target genes in IECs. Our rationale is that an improved understanding of bacterial control of host FA absorption and transcriptional regulatory programs in the intestine could lead to new microbiota-based strategies for controlling fat metabolism and energy balance in humans and other animals. In Specific Aim 1, we will identify host and bacterial mechanisms underlying bacterial stimulation of dietary FA absorption in the intestinal epithelium. In Specific Aim 2, we will define the regulatio and function of Hnf4 in the intestinal epithelium in response to microbial colonization. The expected outcomes will vertically advance the field in several ways. First, they will generate foundational mechanistic insights into how gut bacteria regulate host assimilation of energy-rich dietary fats. Second, they will establish Hnf4 transcription factors for the first time as key mediators of host-microbe commensalism in the intestine. Third, they will provide a novel molecular pathway linking microbial stimulation of host FA absorption to important host transcriptional programs in the intestinal epithelium. These results are expected to have a significant impact because they are likely to lead to new strategies for treating human diseases such as obesity and undernutrition by manipulating dietary fat assimilation and gene expression programs in the intestinal epithelium.